The present invention relates to an isolation valve used in a single crystal pulling apparatus for opening and closing the passage between the main chamber and the upper pull chamber of the single crystal pulling apparatus.
A single crystal pulling apparatus is designed to grow and pull up a single crystal rod from a melt of a polycrystal substance based on the CZ method (Czochralski method). A conventional single crystal pulling apparatus consists mainly of a main chamber, and in this chamber are a quartz crucible for containing the raw material for crystallization, a carbon heater surrounding the crucible to melt down the raw material, a thermal insulator also made of carbon and surrounding the heater, etc. Above the main chamber is provided an upper pull chamber from which the single crystal ingot (or rod) brought up from the main chamber is removed. An isolation valve is provided between the main chamber and the upper pull chamber for opening and closing the passageway between the two chambers. The present invention relates to this isolation valve in particular, of which a preferred mode is depicted in FIG. 1.
In the conventional single crystal pulling apparatus, the growing and pulling operation of the single crystal is conducted in an inert gas atmosphere at a reduced pressure with the view of preventing the precipitation of the silicon oxides which agglomerates on the cold internal walls of the chambers. The oxides are originally formed as a result of the reactions that take place between the melt and the quartz crucible, and thereafter the oxides evaporate from the melt surface into the atmosphere and precipitate on the walls. Some of the oxide agglomerate often causes a disorder in the single crystallization, as it drops into the melt. Also, the operation at a reduced pressure facilitates reduction in consumption of the costly inert gas which is used to prepare the atmosphere. Thus, before the single crystal ingot is pulled up, the main chamber and the upper pull chamber are rendered in full communication with each other by opening the isolation valve, and the chambers are filled with the inert gas at a reduced pressure.
When the growing and pulling operation of the single crystal ingot is completed, the ingot is lifted up into the upper pull chamber and taken away from a hermetically sealable door which is provided in the side wall of the upper pull chamber. But before opening the door, the isolation valve is often closed so that the reduced internal pressure of the main chamber is isolated from the external atmosphere to thereby enable the high temperature of the system in the main chamber to be kept as it is, whereby the next crystallization operation can be started without having to melt the whole of the residual polycrystal substance all over again. Thus, when the door of the upper pull chamber is opened, the internal pressure of the upper pull chamber equilibrates with the atmospheric pressure while the internal pressure of the main chamber is maintained as low as it was when the last pulling operation was conducted.
It is necessary, therefore, to replace the air in the upper pull chamber with the inert gas and reduce the internal pressure to a level as low as that of the main chamber before opening the isolation valve for the next crystallization (pulling) operation. This replacement of the gases is conducted in the following manner: first, the door of the upper pull chamber is closed, and the air in the upper pull chamber is drawn by a vacuum pump until the internal pressure becomes 0.1 millibar (absolute pressure), for example. Then, the inert gas is introduced in the upper pull chamber till the internal pressure thereof becomes 200 millibar, for example. This operation is repeated two to five times so as to entirely replace the air with the inert gas, and at the final time the internal pressure is made equal to the internal pressure of the main chamber, 100 millibar, for example. Then the isolation valve is opened, and the single crystal pulling operation is started.
Now, with reference to FIG. 6 (a) which shows a sealing portion of a conventional isolation valve, the portion corresponding to the portion A of the preferred isolation valve of the present invention shown in FIG. 1, the sealing system of the conventional isolation valve will be explained. An endless seal ring 117 such as an O ring is embedded in a groove which is made in the upper edge of the raised brim of the hole of the passageway that communicates with the main chamber. The element designated by a reference numeral 115 is a shutter corresponding to the element 15 in FIG. 1. Since the shutter 115 is urged downward with an imposed force of 60 kg, for example, by an air cylinder 18 (shown in FIG. 1 and described later) when the isolation valve is closed, the sealing effect is performed by the seal ring 117. In the case of the isolation valve shown in FIG. 6 (b), the endless seal ring 117 is embedded in the lower face of the circular shutter 115 such that the seal ring 117 touches the horizontal upper edge face of the raised brim of the hole that communicates with the main chamber, when the isolation valve is closed. Thus, the sealing of the clearance between the upper pull chamber and the main chamber is effected when the air cylinder 18 urges the shutter 115 downward.
However, when the air in the upper pull chamber is drawn out by the vacuum pump in an attempt to replace the air with the inert gas, the internal pressure of the upper pull chamber is reduced to a pressure much lower than the internal pressure of the main chamber, e.g. 0.1 millibar. Then, the internal pressure of the main chamber pushes the shutter 115 upwardly with a force F proportional to the differential pressure, whereby the shutter 115, overcoming the downward force imposed by the air cylinder 18, lifts itself to a higher position where the increased downward force by the air cylinder 18 counterbalances the upward force F, as shown in FIGS. 6 (a) and (b). Thus lifted, the shutter 115 allows the inert gas to leak from the main chamber to the upper pull chamber, as shown by the arrows in FIG. 6. As a result, an abnormal lowering in the internal pressure of the main chamber takes place, and furthermore the gases in the process of replacement in the upper pull chamber reversely diffuse into the main chamber to thereby contaminate the inert atmosphere therein. Incidentally, in the case of the above example of pressure values, if the diameter d of the seal ring is 250 mm, the uplifting force F would be about 49 kg.